Condensation Heat Transfer Rate Research Articles

Modeling the thermal and hydrodynamic performance of grooved wick flat heat pipes

A compact model is developed to predict the thermal and hydrodynamic performance of a flat heat pipe with a rectangular grooved wick. The present model relies on the analytical solution to the energy equation in the wall and an equivalent heat transfer coefficient predicted using a computationally efficient iterative method. This efficient iterative method can also provide a framework for modeling other grooved or porous wick heat pipes for which analytical or semi-analytical solutions to the wall conduction, fluid flow, and film equations exist. Compared to prior numerical tools, the present modeling approach is computationally efficient, making it compelling for use in parametric and optimization studies. Instead of numerically solving a set of coupled differential equations, the present model considers only analytical and semi-analytical solutions for evaporation and condensation heat transfer rates. The non-discretized nature of the present model allows computations on a typical workstation to be completed within seconds as opposed to the hours required for prevalent numerical tools. The present model accounts for varying liquid fill volumes, geometry, and interfacial properties, such as surface tension and contact angle. The present model closely agrees with published numerical and experimental results for wall temperatures and maximum heat transfer rates. Parametric studies, which vary wall thermal conductivity, water contact angle, and groove dimensions are conducted on a previously experimentally investigated heat pipe to demonstrate the present model’s capabilities. The present model found that the maximum heat transfer rate of the heat pipe can be enhanced by about 15 and 20% by varying its wetting angle and groove dimensions, respectively.

Effect of twisted tapes and surface extensions in enhancing the condensation heat transfer of high-temperature saturated steam in a circumferential rectangular channel

This article numerically investigates the impact of twisted tapes and extended surfaces (fins) on enhancing high-temperature steam flow in a circumferential rectangular channel, akin to the external jacket cavity of a tyre cure press. The passive enhancement techniques for improving the mold warm-up time of the tyre curing process have received limited attention in previous studies, which have primarily focused on enhancing heat transfer in configurations and working conditions typical of heat exchanger applications. The objective of this article is to enhance the condensation heat and mass transfer rate by geometrically modifying the internal flow path of the circumferential rectangular channel, resembling an external jacket cavity of a tyre cure press, using twisted tapes and extended surfaces. Four configurations are simulated: i) conventional circumferential rectangular channel, ii) channel with twisted tapes, iii) channel with surface extension, and iv) channel with both twisted tapes and surface extension. The heat transfer performance of all the modified channel configurations derived from the simulation results is assessed in comparison to the conventional channel. The channel, featuring a design that includes five twisted tapes with a twist ratio of 8 and 54 extended surfaces, records a substantial 36.85% rise in heat flux along with a notable 24% increment in the condensation mass flux. The simulation results are validated using an in-house experimental study. The tyre cure press in the experimental setup is modified to align with the design proposed by the simulation results, and a warm-up study is conducted. The warm-up time for the modified mold to attain the desired temperature is reduced by 33%, with no accompanying increase in steam consumption, compared to the warm-up time for the conventional mold. This significant reduction in mold warm-up time holds immense potential in enhancing the productivity rate, an ongoing requirement of the automobile industry.

Investigating condensation from humid air under mixed convection regime

A wide range of industrial applications rely heavily on heat transfer during vapor condensation from a mixture of water vapor and noncondensable gases (NCG). For a vertically-mounted condenser plate, the vapor-diffusion boundary-layer thickness is influenced by the interplay of the thermogravitational and forced flow fields, eliciting classical mixed convection scenario. This thickness in turn dictates the condensation heat and mass transfer rates. While condensation in presence of NCG under free and forced convection scenarios are well-characterized in the literature, its counterpart in the mixed convection regime is relatively uncharted. Herein, condensation from an upward stream of humid air over a vertically mounted mild steel condenser surface is characterized under different flow velocities. The free-stream flow is thus directed opposite to the thermogravitational flow induced next to the plate. We observe that with increasing the magnitude of the upward flow velocity of the free stream, the condensation heat transfer coefficient (CHTC) initially decreases until it reaches a minimum at 0.4 m/s, beyond which the CHTC rises again with the flow velocity. Using the Nusselt analogy for mixed convection for the relevant flow regimes we substantiate our experimental findings and extend the observation for predicting condensation behavior under different experimental ambient conditions.

Numerical Simulation on Two-Phase Ejector with Non-Condensable Gas

The two-phase ejector is a simple and compact pressure boosting device and widely used in ejector steam-generator water feeding systems and core emergency cooling systems. The direct contact condensation of water and steam is the key process of a two-phase ejector. Usually, the high-temperature and high-pressure steam will inevitably induce non-condensable gases. The existence of non-condensable gases will reduce the condensation heat transfer rate between steam and water, and harm the equipment. This study carried out 3D numerical simulations of a two-phase ejector based on an inhomogeneous multiphase model. The steam inlet pressure and the non-condensable gas mass fraction rang in 0.6–2.9 MPa and 1–10%, respectively. The heat and mass transfer characteristics were analyzed under different conditions. The results show that the heat transfer coefficient and plume penetration length increased with the steam inlet pressure. Non-condensable gas prevents direct contact condensation between the steam and water. The non-condensable gas mass fraction rises from 1% to 10%, the heat transfer between steam and water deteriorates, and leads to a lower heat transfer coefficient.

Reducing CO2 emissions and improving oil recovery through silica aerogel for heavy oil thermal production

Heavy oil reserves comprise more than 60% of global crude oil resources, and thermal oil recovery techniques, especially steam injection, are widely utilized for extracting heavy oil. However, these thermal production processes are characterized by high energy consumption and carbon emissions, which pose significant challenges in reducing CO2 emissions. In response to these challenges, this study presents a novel approach that combines silica aerogel with CO2-assisted thermal recovery technology. Our findings demonstrate that the incorporation of aerogel nanoparticles enhances the adsorption capacity of CO2 in the aqueous phase. The adsorption behavior of CO2 in silica aerogel aligns with the Langmuir model. When silica aerogel nanoparticles are employed in steam flooding, a notable improvement is observed in both oil recovery and CO2 sequestration rate. Molecular dynamics simulations reveal that the porous structure of aerogel enables the adsorption of light saturates molecules from crude oil, promoting their diffusion into the aqueous phase and ultimately enhancing oil recovery. The steam condensation experiments clearly demonstrate that the presence of aerogel and CO2 exerts a substantial influence on the condensation heat transfer of steam, resulting in a reduction in the condensation heat transfer rate between the steam and the cooling wall. Additionally, molecular dynamics simulations reveal that silica aerogel nanoparticles exhibit an affinity for adsorbing onto hydrophobic quartz surfaces, forming an insulating layer between steam and quartz, thereby facilitating CO2 sequestration. The introduction of silica aerogel nanoparticles expands the swept volume of steam heat. This study provides valuable insights into the underlying mechanisms responsible for improved CO2 sequestration and increased oil recovery through the implementation of silica aerogel nanoparticles, offering fresh perspectives on heavy oil recovery.

Sustaining dropwise condensation on nickel-plated copper surfaces with As-grown graphene coatings

• Dropwise condensation on as-grown graphene on electroplated Ni has been achieved. • Dropwise condensation has been successfully promoted on Cu using Ni-Gr coatings. • Condensation rate has been enhanced up to 2.9 times higher than filmwise mode. • The electroplated Ni-Gr coatings on Cu are more robust than the Ni-Gr surface. Dropwise condensation (DWC) has been extensively investigated owing to its substantially higher efficiency compared with filmwise condensation (FWC). However, performing the highly efficient DWC has been hindered by the durability of existing hydrophobic surface coatings. Graphene grown on Nickel (Ni-Gr composite) has been demonstrated in our previous study to sustain the DWC in an aggressive condition for more than 36 months. Compared with traditional heat exchangers made of copper (Cu) and stainless steel, the high purity requirement of Ni substrate for graphene growth and high material cost greatly limit its practical applications. In this study, the cost-effective nickel-plating technique has been explored to apply the robust Ni-Gr coating on prevail metals such as Cu in industry. DWC can be sustained for more than 180 days without noticeable degradation in a steam/air mixture environment. After 180 days, the Ni-Gr coating electroplated on the copper surface (Cu-Ni-Gr) can still facilitate DWC with approximately 2.9 times higher condensation heat transfer rate than that of conventional metal surfaces. The consistently superior anticorrosion properties of Cu-Ni-Gr surfaces is further revealed by Tafel tests. An additional 46% lower corrosion rate than the Ni-Gr surface has been characterized. This study manifests a feasibility to implement the sustainable and highly efficient DWC on other popular heat exchanging metals using super-durable Ni-Gr coatings.

Numerical investigation on direct contact condensation-induced water hammer in passive natural circulation system for offshore applications

For the passive natural circulation systems (PNCSs) in offshore floating nuclear plants, the ocean environment is usually regarded as the final heat sink. Condensation-induced water hammer (CIWH) may occur as the saturated steam released into the ocean by natural circulation. This study aims to develop a CFD model to analysis the CIWH phenomenon induced by direct contact condensation (DCC) in the PNCSs. In the model framework, the two-phase interface is tracked explicitly by the volume of fluid method; the interface heat transfer is acquired by the hybrid Heat Transfer Coefficient model based on the Surface Renewal Theory; the condensation heat and mass transfer rate are obtained by user-defined functions (UDF). The condensation model is firstly validated with the PMK-2 experimental data, showing consistency with the measured temperature evolutions. Then, the model is applied to investigate the CIWH events in the horizontal pipe geometry of the passive natural circulation systems. The simulation results indicate that the reverse flow of subcooled water triggers the formation of the CIWH events, by increasing the condensation rate and forming the entrapped bubble. The reverse flow periodically occurs afterwards, inducing multiple CIWHs. The increase of inlet stream velocity lengthens the reverse flow period and the distance, thus strengthening the interfacial condensation and the pressure impulse of CIWH.

Exploring the limits of condensation heat transfer: A numerical study of microscale-confined condensation between parallel surfaces having wetting contrast

Dropwise condensation has received extensive interest due to its potential to enhance the efficiency of power generation, water harvesting, and thermal management systems. The degree of heat transfer enhancement during dropwise condensation is hindered by the delicate balance between nucleation, droplet growth rate, and condensate shedding. Condensation between parallel surfaces having wetting contrast, termed microscale-confined condensation (MCC), presents a promising method to break the fundamental limits of dropwise condensation by decoupling droplet nucleation and growth from condensate removal. The key mechanisms governing MCC enable spontaneous condensate transfer from the condensing surface to a highly wetting absorbing surface to limit the maximum droplet size. Here, we developed and explored numerical simulations of MCC to elucidate the upper limits of the condensation heat transfer rate. To optimize MCC with respect to heat transfer, we examined the effects of surface gap or maximum droplet size, and condensing surface contact angle on droplet size distribution and overall condensation heat flux. We then developed a regime map in terms of surface wettability and shedding droplet size, and showed that smaller surface gaps and smaller condensing surface contact angles are preferential to achieve a higher heat flux in pure vapor condensation conditions. The optimized MCC outperforms state-of-art condensation including classic dropwise condensation and jumping-droplet condensation, with a 250% enhancement of heat transfer coefficient at a steam temperature of 100 °C. Through numerical simulations, our study not only calculates the upper bound of condensation heat transfer, but also provides guidelines for further enhancement of condensation via decoupling droplet shedding from nucleation.

A neural network model for free-falling condensation heat transfer in the presence of non-condensable gases

Condensation heat transfer has been widely studied for various applications such as power generation, water desalination, data centers, chemical and pharmaceutical syntheses, heating and air conditioning systems, and especially, the passive containment cooling system (PCCS) in nuclear power plants. The PCCS is designed to reduce the risk resulting from a loss of AC power or compounding human error. In a hypothetical accident situation, one of the main safety systems, the PCCS, condenses water vapor with gravity-driven force to remove the heat from the containment vessel. Although the condensation heat transfer in the presence of non-condensable gases for predicting the heat transfer performance of the PCCS has been successfully investigated, there is still no generalized model or correlation available. In this work, we focused on the development of universal models for predicting the heat transfer coefficients of free-fall condensation heat transfer on the external surfaces of vertical tubes in the presence of non-condensable gases. For this, we created a consolidated database covering a broad range of geometric values and operating conditions, including tube hydraulic diameter Dh = 10–41.2 mm, tube length L = 0.3–3.5 m, total pressure Ptot = 0.15–2.0 MPa, wall subcooling temperature ΔTsub = 5–71 K, average condensation heat transfer coefficients of 90 ≤ ͞h ≤ 19,400 W/m2K, and Reynolds numbers of the film of 8 ≤ Refilm ≤ 7160. We analyzed the influence of varying the geometric and operational parameter values by using this database. Conventional machine learning techniques such as nonlinear regression and multilayer perceptron (MLP) neural network methods were adopted to predict the condensation heat transfer rate based on the consolidated database. Moreover, the prediction accuracies of the condensation heat transfer rates of the proposed prediction models and 12 relevant correlations were compared by using the consolidated database. The proposed nonlinear regression model exhibited good prediction accuracy with a mean absolute error (MAE) of 12.7 % for average ͞h, which is much lower than those achieved by previously proposed relevant correlations. In addition, the MLP neural network model showed excellent prediction accuracy with an MAE of 4.2 % for the consolidated database.

Grain size effects on the wettability of as-grown graphene and dropwise condensation

Understanding the wettability of graphene is both fundamentally important and crucial for development of graphene based electronic/optical devices. However, graphene wettability characterized by the static contact angle (CA) has been inconsistent and even contradictory in literature. After systematically characterizing the wettability of chemical vapor deposited polycrystalline graphene, we learn that the wettability of polycrystalline graphene on copper (Cu-Gr) varies significantly with the average graphene grain areas. With the average graphene grain areas increasing from 0.47 μm2 to 22.16 μm2, the static CA on polycrystalline graphene surfaces increases from 47.9° to 68.8°, while the hysteresis CA decreases from 45.9° to 32.7°. Pinning contact lines of nucleated micro-scale droplets along the graphene grain boundary (GB) were identified, indicating inhomogeneous wetting. To further verify the impact of graphene GB on the graphene wettability and manipulating droplet dynamics, we conducted water vapor condensation experiments on the Cu-Gr surfaces. We found that a larger graphene grain size (less graphene GBs per unit area) would lead to a higher droplet departure frequency, resulting in an additional 45% enhancement of dropwise condensation (DWC) heat transfer rates. This study successfully quantifies the role of graphene grain size in determing CAs and reveals that the graphene surface wettability and DWC performance can be effectively engineered by varying graphene grain size.

Feasibility analysis for intermediated fluid type LNG vaporizers using R32 and R410A considering fluid properties

Abstract LNG vaporizer is widely used all over the world to vaporize LNG by using seawater. An intermediated fluid type vaporizer (IFV) uses intermediated fluid between LNG and heat source to transfer heat, and is enable to utilize LNG cold energy for a cryogenic power generation, and so on. Conventionally propane has been used as an intermediated fluid; however, refrigerants other than propane are recently needed because of the diversified usage environments. In this study, R32 and R410A are selected and their feasibility is discussed in heat transfer aspect. By using a number of unit (NTU) method, prediction of boiling and condensation heat transfer rate in tube bundles was carried out. The prediction demonstrated that those selected refrigerants can increase the vaporization rate with IFV from the base line of propane. From those, feasibility of refrigerant use in IFV was shown.

Enhancing filmwise and dropwise condensation using a hybrid wettability contrast mechanism: Circular patterns

Condensation surfaces using a wettability contrast mechanism is one of the effective methods to enhance heat transfer rate. The pattern design plays a critical role in realizing various contrast degrees of wettability. In this study, water vapor condensation of two series of hybrid circular-patterned designs were developed and experimentally characterized under the atmospheric pressure with the presence of non-condensable gasses (air). The outer surfaces of copper tubes in the horizontal orientation were used as the condensation surface with the designed patterns. The condensation heat transfer rates were compared to that of the complete dropwise condensation (DWC) and complete filmwise condensation (FWC), respectively. In the first series, the pattern design consists of hydrophobic (β) circular patterns formed on a less-hydrophobic (γ) background. The diameter of the patterns was varied to find an optimum configuration, which was observed with the pattern diameter and pattern distance at 1.5 mm and 0.5 mm, respectively. Significantly enhanced condensation rate was obtained compared to FWC. In the second series, the design consists of γ-patterns contrasting with a β-background, which is an inverse design of the first series in terms of wetting conditions. A parametric study was conducted to examine the influence of the pattern dimensions on the condensation. In the second series pattern designs, significant enhancements compared with both FWC and DWC were achieved. The optimal pattern was found to be at a diameter of 1.5 mm and gap of 1.5 mm, leading to a 79% higher heat transfer rate compared to that of the complete DWC.

Quantifying interfacial parameters of upward and downward annular flow condensation from high-speed visualization

Condensation of R-134a in upward and downward annular flow was experimentally studied in a 5-mm ID transparent test section using a LED-based image processing technique. In addition to the flow orientation, the frequency of the film structures was quantified in terms of the following parameters: total mass flow rate, condensation heat transfer rates (outlet vapor quality) and test section inclination with respect to the horizontal. The image processing technique combined Fourier analysis and the Kernel density estimation technique to calculate the two-dimensional (amplitude–frequency) probability density function of the film flow structure as a function of the independent parameters. Although the tube inclination and the condensation rate showed some influence on the frequency of the interfacial waves, the most significant effect was due to the flow orientation, which resulted in abrupt frequency changes apparently linked to flow regime transitions dictated by shear and gravity forces.

Experimental investigation on a novel sewage-resource-based system with functions of heat recovery and water purification

Sewage source heat pump was a promising technology in energy saving. To solve the fouling accumulation on sewage heat exchangers, a novel sewage-resource-based system with sewage tower equipped with guide plates was proposed to realize heat recovery and water purification. Circulating air extracted heat from sewage in the core component of sewage tower through evaporating some of the sewage, and released heat and formed condensate on evaporator surface of heat pump. Results indicated that the heat transfer rate in condenser improved from 2.629 kW to 4.876 kW and the coefficient of performance of heat pump unit (COPu) promoted from 3.02 to 5.01 with sewage temperature increased from 9.44 °C to 31.41 °C. COPu and the coefficient of performance of system (COPs) initially increased sharply and then decreased slightly with increase of the volume flow ratio of air to sewage in the sewage tower, and the turning point occurred near the volume flow ratio of air to sewage of 359:1. In addition, compared with sewage tower with baffle structure, the novel sewage heat pump system with guide plates had greater heat transfer rate of 5.71 kW at 1000 m3/h and lower air-side pressure drop in sewage tower. Meanwhile, ammonia-nitrogen concentration in the condensate was 14.72 mg/L which was 63.64% lower than that in the original sewage, indicating the water quality of condensate satisfied the requirements for urban greening and construction articulated in the water quality standard.

Cost Benefit Analysis Of Window Air Conditioning System with Evaporative Cooled Condenser

This paper shows performance analysis of the possibility of augmentation of Coefficient of Performance and reduced power consumption of a window air conditioning trainer using an evaporatively cooled condenser. In general, during the dry and hot season, performances of without evaporative cooling condenser minimize, and Coefficient of Performance decreases because of condenser heat transfer rate decreases. This hypothesis has been tested by setting an experimental set up of a window air conditioning trainer using evaporative cooling for condensation. The results show a considerable enhancement of Coefficient of Performance of with evaporative cooling condenser as compared to without evaporative cooling condenser. The test however also shows the applicability of evaporative cooling only during the torrid atmospheric conditions.

Condensation Droplet Distribution Regulated by Electrowetting

Abstract This paper presents a visualization of condensation droplet distribution affected by the electrowetting-on-dielectric (EWOD) approach. A single-side double-layer-electrode design (grid wire, thin wire, and thick wire) and coplanar-electrode design (zigzag) are discussed. Side-by-side experiments with applied 40 V DC electric potential are carried out to compare droplet distribution between identically designed charged and uncharged devices. The uncharged devices show a random droplet distribution, whereas charged devices have a regulated distribution based on the designed patterns. As droplets on the electrode boundaries become larger, they are likely to slide away and stay in electrode-free regions. The droplets “sit” inside the grid wires and distribute vertically along thin and thick wires. On the coplanar-electrode zigzag device, droplets are distributed vertically. The charged surfaces lead to a faster droplet growth rate and more dispersed droplet distribution. This phenomenon accelerates the shedding frequency of the droplets and frees up more areas for small droplets to nucleate and grow. The first shedding moment of the charged surfaces occurs earlier than the uncharged ones for all types of EWOD devices. The detected droplet shedding diameter ranges from 1.2 mm to 2.5 mm in this study. The number of large droplets is found greater on the charged devices compared with the uncharged devices and theoretical model. The work presented in this paper introduces a novel approach to actively influence droplet distribution on microfabricated condensing surfaces and indicates great potential for improving the condensation heat transfer rate via EWOD.

Background Oriented Schlieren (BOS) imaging of condensation from humid air on wettability-engineered surfaces

Condensation of water vapor in presence of non-condensable gas is observed in several engineering applications ranging from power generation to water harvesting. Non-intrusive optical diagnostics of the thermal and vapor concentration boundary layers on a condensing surface can offer a veritable means of predicting the condensation heat transfer coefficient (CHTC), but the literature in this field is far from being well-developed. Here we perform a Schlieren imaging-based mapping of the thermogravitational density boundary layer during condensation from humid air on vertical surfaces of three different wettability, i.e., superhydrophilic, hydrophilic (control) and superhydrophobic. A lens-free, Background Oriented Schlieren (BOS) technique is used to cross-correlate the refraction of light and quantify the density gradient fields near the condenser plates for three different humidity ratios (13.5, 15.5 and 17.5 g/kg dry air) at a constant degree of subcooling (19.5 °C). CHTC measurement from condensate collection data is used to validate the non-intrusive data. The mean refraction angle and the mean density gradient near the plate are found to scale linearly with the CHTC and the condensate mass flux, respectively. The study clearly shows the potential of BOS technique in predicting the condensation heat transfer rates from humid air.

Modeling and Simulation of Dropwise Condensation: A Review

In this review, we present significant developments that have been made in the mathematical modeling and simulations of dropwise condensation. In dropwise condensation, vapor condenses in the form of distinct drops. Modeling of DWC involves modeling heat transfer through a single drop and applying it to a population of drops. In the first part, we discuss heat transfer through a single droplet and compare the approximate analytical solution with the results of numerical simulations. We also address the shortcomings of the analytical model. In the second part, we present methods utilized to find the drop size distribution which are coupled with a model for heat transfer through the single droplet to obtain overall dropwise condensation heat transfer rate. In particular, we discuss the population balance method and the Monte Carlo method to predict drop size distribution and heat transfer rate. We support our discussion with the results from the literature.

Thermal fluid characteristics of pulsating heat pipe in radially rotating thin pad

The thermal performances of a radially rotating pulsating heat pipe (RPHP) formulated by the interconnected 1 × 4 mm2 channels in the thin pad were investigated. With the 50% filling ratio of water and the specific geometric configurations, the selected parameters controlling the thermal-fluid phenomena by the present study were boiling number (Bn), dimensionless centrifugal acceleration (Ω) and thermal resistance of condenser (Rth,cond). The vapor-liquid flow images, heat transfer rates and thermal resistances of the pad-type RPHP were detected at the rotor speeds of 0, 100, 300 and 500 rev/min. The matrix of test conditions was formulated by four Bn, four Rth,cond and four Ω for both sets of flow visualization and heat transfer tests. As Ω increased from 0 to 27.8, the intermittent vapor slugs gradually deformed toward the tiny bubbles to incur the respective Nusselt number reductions and elevations over the evaporator and condenser of the RPHP. As the dominance of condenser heat transfer rate in constructing the overall thermal resistance (Rth) took precedence to its evaporator counterpart, the increases of Ω led to the Rth reductions for the present RPHP. The empirical correlations using Bn, Ω and Rth,con as the controlling parameters were devised to evaluate Rth and the regionally averaged Nusselt numbers over the evaporator and condenser of the present RPHP for relevant applications.

Heat transfer during condensing droplet coalescence

Dropwise condensation can yield heat fluxes up to an order of magnitude higher than filmwise condensation. Coalescence is the primary mode of growth for condensing droplets above a small threshold size (e.g., radius r > 2 μm for water at 1 atm), but no prior studies have quantitatively assessed heat transfer during coalescence. Previous models of dropwise condensation have generally described coalescence as an instantaneous event, with a step reduction in heat transfer rate. However, coalescence and recovery of a quasi-steady droplet temperature profile requires a finite time, during which the direct droplet condensation heat transfer rate gradually decays. Additionally, during this period, the droplet may oscillate, repeatedly clearing the surrounding surface and resulting in high overall heat fluxes. This study employs Volume-of-Fluid (VOF) simulations to quantitatively assess these two transient heat transfer processes during droplet coalescence. It is shown that the direct mechanism of gradual heat transfer decay can be represented by a decaying exponential function with a time constant τ. Simulations are performed to determine τ(r1,Rt) for (1μm⩽r1⩽25μm;1⩽Rt⩽4) where r1 is the radius of the smaller droplet and Rt is the radius ratio between the two merging droplets. For water at atmospheric pressure this spans the range of droplet sizes through which most of the heat transfer occurs on a surface (∼80%). A simple correlation is proposed for τ(r1,Rt) for the studied droplet size range, fluid properties, and surface conditions. These simulations are also employed to determine the order of magnitude of heat transfer enhancement due to repeated clearing of the surrounding surface as droplets coalesce. Findings can inform improved models of dropwise condensation that more accurately predict transient heat transfer during coalescence events.